Recent internal combustion engines have various exhaust cleaning devices such as an oxidizing catalyst, NOx catalyst or three-way (oxidizing and reducing) catalyst. Diesel engines have diesel particulate filters (DPF), as an exhaust cleaning device, for collecting particulates in the exhaust. The DPF is regenerated by burning the collected particulates. The DPF carries oxidizing catalyst thereon so that the particulates are burned stably under low regenerating temperatures.
For an efficient operation, the temperature of the exhaust cleaning device should preferably be maintained in a temperature range (200° C.-700° C.) in which the catalyst is maintained sufficiently activated and no catalyst damage due to excessive temperatures is caused. For regenerating the DPF, unburned HC is supplied by post injection to raise the DPF temperature to be above 600° C. The DPF may become excessively heated as a result of burning of the particulates, resulting in deterioration of the catalyst or damage to filter components.
Various catalyst monitoring devices are proposed. For instance, in U.S. Pat. No. 5,211,010 (JP-A-4-224221), an exhaust temperature sensor is provided downstream an NOx reducing catalyst and regulates catalyst temperatures by increasing or decreasing air excess ratio in the exhaust based on the detected exhaust temperature. Further, in JP-A-1-216009 (JP 2593506), exhaust temperature sensors are provided both upstream and downstream a catalyst and determines catalyst deterioration based on a comparison of the detected catalyst upstream and downstream temperatures. The catalyst or DPF monitoring will become impossible if the exhaust temperature sensors fail to operate normally.